Presentation on theme: "The North Atlantic − The NAO, AO and the MJO"— Presentation transcript:
1The North Atlantic − The NAO, AO and the MJO Hai LinMeteorological Research Division, Environment CanadaWorkshop “Sub-seasonal to Seasonal Prediction”Met Office, Exeter, Dec. 1-3, 2010
2Outlines Challenge of prediction in the North Atlantic and Europe Brief introduction of NAO / AO and its impactNAO prediction on intraseasonal time scaleMJO contribution;intraseasonal hindcastNAO seasonal predictionpossible signal sources;skill in four Canadian AGCMs
3Challenge of subseasonal and seasonal prediction in the North Atlantic and European region Strong variability due to atmospheric internal nonlinear interactionsFar from major source of interannual variability (e.g., ENSO)Low forecast skill
4What is the NAO?The North Atlantic Oscillation is a large-scale seesaw in atmospheric mass between the subtropical high-pressure system over the Azores Islands and the subpolar low-pressure system over Iceland.(From American Museum of Natural History website)
5The NAOThe NAO is one of the most important modes of atmospheric variability in the northern hemisphereThe NAO has a larger amplitude in winter than in summerThe NAO accounts for 31% of the variance in winter surface air temperature north of 20°N (Hurrell, 1995)
6The AOThe Arctic Oscillation has a global scale, more zonally symmetric, also called the Northern Annular Mode (NAM)Connection to stratosphere (e.g., Baldwin and Dunkerton 2001)The NAO can be regarded as a local representation of the AO in the North Atlantic
7Impact of the NAO Subtropical high pressure and Icelandic low Westerly winds and storm activity across the Atlantic OceanTemperature and precipitation in Europe, northeastern Canada and GreenlandImpact on forecast skill
8How is the NAO variability generated? Causes within the atmosphere: interactions among different scales and frequencies in the atmosphere lack of forecast skill beyond 2 weeksCauses external to the atmosphere(on seasonal and interannual time scales):Sea surface temperature (SST) anomaly in the North AtlanticChanges in ice and snow coverSST anomaly in the tropics
9NAO forecastsIntraseasonal time scaleimpact of the MJO
10The Madden-Julian Oscillation (MJO) Discovered by Madden and Julian (1971). Spectrum analysis of 10 year record of SLP at Canton, and upper level zonal wind at Singapore. Peak at days.Dominant tropical wave on intraseasonal time scale30-60 day period, wavenumber 1~3propagates eastward along the equator (~5 m/s in eastern Hemisphere, and ~10 m/s in western Hemisphere)Organizes convection and precipitation
11Composites of tropical Precipitation rate for 8 MJO phases, according to Wheeler and Hendon index.Xie and Arkin pentad data,The red arrows mark Phases 2-3 and Phases 6-7. They are characterized by a dipole convection anomaly distribution in the tropics. As will be shown later, such a dipole diabatic heating has a great impact on the Northern Hemisphere extratropics and the NAO.
12Connection between the MJO and NAO NAO index: pentad averageMJO RMMs: pentad averagePeriod:Extended winter, November to April (36 pentads each winter)
13Lagged probability of the NAO index Positive: upper tercile; Negative: low tercile Phase12345678Lag −5−35%−40%+49%Lag −4+52%+46%Lag −3Lag −2+50%Lag −1Lag 0+45%−42%Lag +1+47%−46%Lag +2+42%−41%Lag +3+48%−48%Lag +4−39%Lag +5From left to right: 8 phases of the MJO. From top to bottom: lags in pentads. Negative lags mean that the NAO leads the MJO, while positive lags indicate that the NAO lags the MJO.The numbers are probability of occurrence of strong positive NAO (red) and strong negative NAO (blue). Those passing 5% significance level (bootstrap method) are shown.Here we look at lags > 0 for impact of the MJO on the NAO. 1-3 pentads after MJO Phases 2-4, there is high probability of strong positive NAO. 1-4 pentads after MJO Phases 6-8, high chance of strong negative NAO.For lags < 0, it is the impact of NAO on MJO, we are not focus on this aspect in this talk.(Lin et al. 2009)
14Tropical influence Z500 anomaly (Lin et al. JCLIM, 2009) Composite of Z500 anomaly after MJO Phase 3 (upper panels), and Phase 7 (lower panels). Phase 3 has a dipole tropical convection anomaly (above normal convection in Indian Ocean and below normal convection in western Pacific). So does Phase 7, but with an opposite sign.At the simultaneous composite of Phase 3 (a), a negative PNA-like pattern is generated. One pentad later (b), Rossby wave dispersion can be seen with intensification of downstream centers. After another pentad (c), a positive NAO is formed. There is some contribution from transient eddies in the last stage.For Phase 7, the Rossby wave propagation is less clear. A negative NAO is generated at lag=2 pentads.(Lin et al. JCLIM, 2009)
15Impact on Canadian surface air temperature Lagged winter SAT anomaly in CanadaThis is an example of the impact of the MJO on surface temperature. Such signal would be useful for intraseasonal prediction in North America.(Lin et al. MWR, 2009)
16Why the response to a dipole heating is the strongest ? Barotropic instability of 2-D basic flow: similar mechanism as Simmons et al. (1983)Rossby wave generation determined by relative position of tropical forcing wrt jet stream (Lin 2010)To demonstrate this:Primitive equation GCM (T31, L10)Linear integration, winter basic statewith a single center heating sourceHeating at different longitudes along the equator from 60E to 150W at a 10 degree interval, 16 experimentsZ500 response at day 10These two points are possible mechanisms.
17Similar pattern for heating 60-100E a) 80E Day 10 Z500 linear responseSimilar pattern for heating Ea) 80Eb) 110EWhen there is a dipole as MJO Phase 3 (heating at 80E and cooling at 150E), (note for cooling the response of c reverse sign), a-c makes a very strong extratropical response.Similar pattern for heating Wc) 150ELin et al. (2010)
18ISO hindscast with GEMGEM clim of Canadian Meteorological Centre (CMC)--GEMCLIM 3.2.2, 50 vertical levels and 2o of horizontal resolution3 times a month (1st, 11th and 21st)10-member ensemble (balanced perturbation to NCEP reanalysis)NCEP SST, SMIP and CMC Sea ice, Snow cover: Dewey-Heim (Steve Lambert) and CMC45-day integrationsGEM refers to the Global Environmental Multi-scale model, which is an operational model of Canadian Meteorological Centre.
19NAO forecast skill extended winter – Nov – March tropical influence A simple measure of skill:temporal correlation of NAO index btw forecast and observations
20The dynamical model behaves clearly better than the persistence forecast. Skill above 0.5 up to the 3rd pentad.(Lin et al. GRL, 2010)
21The solid lines are skill of ensemble forecast The solid lines are skill of ensemble forecast. Red line is for those forecasts whose initial condition has a strong MJO (amp >1), whereas the blue line for those with a weak MJO (amp <1) in the initial condition. The dashed lines are average of individual members. The vertical bars are spread among members. The amp of MJO is defined as square root of (RMM12+RMM22).It is clear that an NAO forecast starting from a strong MJO has better skill.(Lin et al. GRL, 2010)
22NAO forecasts grouped by the MJO phase at the initial condition NAO forecasts grouped by the MJO phase at the initial condition. Red line corresponds to those with a dipole convection anomaly.(Lin et al. GRL, 2010)
23Correlation skill: averaged for pentads 3 and 4 When the initial condition has a strong MJO, there is a better forecast skill (15-20 days). The pattern reflects the NAO.Correlation skill: averaged for pentads 3 and 4
24Correlation skill: averaged for pentads 3 and 4 This also leads to improved skill in surface temperature over North Atlantic and European region.(Lin et al. GRL, 2010)
25NAO seasonal forecasts Possible signal sources:Sea surface temperature (SST) anomaly in the North Atlantic (e.g., Rodwell et al. 1999)Changes in ice and snow cover (e.g., Cohen and Entekhabi 1999)SST anomaly in the tropics (e.g., Jia et al. 2008)Now we switch to seasonal forecast of the NAO.
26Historical forecast (HFP2) 4 global models GEM: 2°x2°, 50 levelsAGCM2: 625 km (T32), 10 levelsAGCM3: 315 km (T63), 32 levelsSEF: 210 km (T95), 27 levelsOnce a month (beginning of each month)4-month integrations10 members each modelPersistent SST anomalySea ice and snow cover anomalies relaxed to climatology
27Skill of seasonal mean NAO index as a function of starting month Skill of seasonal mean NAO index as a function of starting month. The horizontal line represents 5% significance level.It is seen that for lead=0 forecast, late winter and spring forecasts are skillful. Consistent among 4 models.
28The skill drops to non-significant for lead=1 month forecasts The skill drops to non-significant for lead=1 month forecasts. There is no skill for NAO seasonal forecast if skipping month 1.
29NAO seasonal forecast skill Lead=0: skill in late winter to springFour models have similar performanceLead=1 month: no skillPossible explanation:skill comes from initial conditionmodels do not have a correct response pattern in the NAO (this will be explored in the next couple of slides)
30Identify dominant forced patterns For the DJFM run:SVD analysis between November tropical Pacific SST andDJF or JFM ensemble mean Z500The expansion coefficient of SVD2 (Z500) is significantly correlated with the observed NAO indexAs for DJFM run, November SST anomaly is used, so we use November SST for the SVD analysis. Ensemble mean of Z500 forecast represents the forced component. The SVD will identify the source and response in the model.
31Leading pairs of SVD in observations November SST vs JFM z500Z500SVD1 (left panels) represents a typical ENSO signal.SVD2 (right panels): positive NAO in Z500, and negative SST anomaly in central equatorial PacificThis is from observations.SST
32Leading pairs of SVD in GEM ensemble mean November SST vs JFM z500For GEM model (ensemble mean)SVD1 very well reproduce the observed ENSO signal.SVD2: Z500 is very different from the NAO. Therefore the model response to the second SVD SST has a wrong spatial pattern.
33Leading pairs of SVD in GCM3 ensemble mean November SST vs JFM z500The same feature as in GEM can be seen for GCM3.
34NAO skill of ensemble forecast Temporal correlation with DJF observed NAO indexForecast NAO indexForced SVD2GCM2-0.130.30GCM30.260.57SEF0.330.47GEM0.250.39Although the model SVD2 has a biased spatial pattern (from NAO), its time evolution is significantly correlated with the observed NAO.This is for lead=0.Lead = 0
35NAO skill of ensemble forecast Temporal correlation with JFM observed NAO indexForecast NAO indexForced SVD2GCM2-0.310.35GCM30.270.43SEF0.120.42GEM0.200.31This is also true for lead=1 month.Lead = 1 month
36NAO skill of ensemble forecast Model has a biased NAO patternThe forced SVD2 pattern has a time evolution that matches well the observed NAO index can be used as a skillful forecast of the NAO indexThe biased pattern is likely caused by bias in model climatology, as the Rossby wave propagation and Atlantic response depends on basic state. However, the time evolution of the signal source can provide some useful information for seasonal forecast of the NAO index.
37Summary Significant impact of the MJO on the NAO NAO intraseasonal forecast skill influenced by the MJOSome skillful NAO seasonal forecast possible in late winter and springSeasonal forecast of NAO has biased spatial pattern, some statistical post-processing procedure can improve the skill